A highly efficient procedure for transferring functional genes into mammalian cells has been developed using retroviral vectors as a delivery system. When mouse bone marrow cells are infected in vitro with a NeoR gene and reinjected into a lethally irradiated recipient mouse, 85-90% of the stem cells (CFU-S) can be shown to carry an intact copy of the NeoR gene. The majority of these cells produce the NeoR gene product: phosphotransferase. Retroviral vectors containing the human gene for the enzyme adenosine deaminase (ADA) as well as the NeoR gene have been made. Using the knowledge obtained from the murine system a non-human primate autologous bone marrow transplantation/gene transfer protocol has been developed. Low levels of both the human ADA gene and the NeoR gene have been expressed in the peripheral blood cells of sever monkeys. In utero gene transfer has also been successfully accomplished using a fetal sheep marrow transplantation protocol. In addition, these vectors have been used to introduce exogenous genes into human bone marrow progenitors in vitro. These studies are preliminary to attempting human gene therapy in patients suffering from ADA severe combined immunodeficiency disease.